亚热带次级森林演替过程中模拟氮磷沉降对土壤微生物生物量及土壤养分的影响

氮（N）沉降深刻影响着森林生态系统的生物多样性、生产力和稳定性。亚热带地区森林土壤磷（P）的有效性较低，N沉降将更突显P的限制作用。N、P输入对亚热带次级森林土壤的影响是否依赖于森林演替阶段知之甚少。选取两种不同演替年龄阶段（年轻林：<40 a；老年林：>85 a）的亚热带常绿阔叶林，设置模拟N和/或P沉降（10 g m^-2 a^-1）4个处理（Ctrl、N、P、NP），连续处理4.5年后采集表层、次表层和下底层（0-15、15-30、30-60 cm）土壤样品，综合分析了土壤微生物生物量碳（MBC）氮（MBN）和多种土壤养分含量。结果表明，MBC、MBN及土壤养分含量均随土壤深度增加而降低。N添加对两种演替阶段森林土壤中MBC和MBN均无显著影响。施P相关处理（P和NP）对年轻林表层土壤MBC和MBN无显著影响，但显著增加了老年林表层土壤MBC和MBN（P<0.05），表明老年林可能比年轻林更易受P限制。N添加显著增加了两种演替森林表层土壤可溶性有机氮（DON）、氨态氮（NH₄⁺-N）和硝态氮（NO₃^--N）的含量（P<0.05）；P相关处理（P和NP）显著增加两种演替阶段表层和次表层土壤速效磷（AP）以及表层土壤全磷（TP）的含量（P<0.05）。土壤MBC和MBN与土壤中各养分指标（可溶性有机碳DOC、DON、NH₄⁺-N、NO₃^--N、AP、全碳TC、全氮TN和TP）呈显著正相关关系，土壤TC、TN和DOC是影响土壤微生物生物量的主要因子。研究可为评估和揭示未来全球环境变化背景下不同演替林龄亚热带森林的土肥潜力及土壤质量的演变提供一定的科学理论依据。

Impacts of simulated nitrogen and phosphorus depositions on soil microbial biomass and soil nutrients along two secondary succession stages in a subtropical forest

Subtropical forest as one of the most typical forest ecosystems in China has rich species diversity of plants, animals and microorganisms. Global environmental changes can influence biodiversity and ecological function in the subtropical region. For example, atmospheric nitrogen (N) deposition profoundly affects the biodiversity, productivity and stability of forest ecosystems. Phosphorus (P) availability in subtropical forest soil is generally low, resulting in more exacerbated P limitation under N deposition scenarios. Soil microbes play key roles in organic matter decomposition and biogeochemical N and P cycling in soils and regulating forest succession. However, the effects of N and P inputs on forest soil characteristics at different succession stages remain largely unknown. A simulated N and/or P deposition experiments (10 g m^-2 a^-1, N addition (N), P addition (P), NP, and Control (without NP addition)) were conducted in a Chinese subtropical forest with two different succession stages (young forest < 40 and old forest > 85 a) since 2015. Soil samples were collected from three soil layers (0-15, 15-30, 30-60 cm) under four N and/or P addition treatments of two forest succession stages in June 2019. Soil microbial biomass carbon (MBC) and nitrogen (MBN), and soil nutrients content were determined through the CHCl₃ fumigation extraction and chemical analysis methods, respectively. The results indicated that soil MBC, MBN, and nutrients decreased with increasing soil depth regardless of the forest succession stages. Nitrogen addition had a minor effect on soil microbial biomass. Although no significant P addition effect was observed in the surface soil of young forest, P alone and NP additions significantly increased MBC and MBN in surface soil samples collected from the old forest (P<0.05), indicating that old forest would be more susceptible to P limitation than young forest. Soil dissolved organic nitrogen (DON), NH₄⁺-N, and NO₃^--N in the surface soils of the old and young forests were significantly increased under N addition conditions (P<0.05). Moreover, P and NP additions significantly increased available P (AP) in the surface and subsurface soils, and total P (TP) content in the surface soil of the two forest succession stages (P<0.05). Significant positive correlations between soil microbial biomass with soil nutrients were observed in this study. Soil total carbon (TC), total nitrogen (TN) and dissolved organic carbon (DOC) were identified as main factors influencing soil microbial biomass. This study provided basic data for revealing the response of forest soil qualities at different succession stages under future global environmental change scenarios.